Drilling in deep waters or drilling through depleted reservoirs is a challenge due to the narrow margin between the pore pressure and fracture pressure. The narrow margin implies frequent installation of casings, and restricts the mud circulation due to frictional pressure in the annulus. Low flow rate reduces drilling speed and causes problems with transport of drill cuttings in the borehole.
Normally, two independent pressure barriers between the reservoir and the surroundings are required. In a subsea drilling operation, normally, the primary pressure barrier is the drilling fluid (mud) column in the borehole and the Blow Out Preventer (BOP) connected to the wellhead as the secondary barrier.
Floating drilling operations are more critical compared to drilling from bottom supported platforms, since the vessel is moving due to wind, waves and sea current. Further, in offshore drilling the high pressure wellhead and the BOP is placed on or near the seabed. The drilling rig at surface of the water is connected to the subsea BOP and the high pressure wellhead with a marine drilling riser containing the drilling fluid that will transport the drilled out formation to the surface and provide the primary pressure barrier. This marine drilling riser is normally defined as a low pressure marine drilling riser. Due to the great size of this riser, (normally between 14 inch to 21 inch in diameter) it has a lower internal pressure rating than the internal pressure rating requirement for the BOP and high pressure (HP) wellhead. Therefore, smaller in diameter pipes with high internal pressure ratings are running parallel to and being attached to the lower pressure marine drilling riser main bore, the auxiliary HP lines having equal internal pressure rating to the high pressure BOP and wellhead. Normally these lines or pipes are called kill and choke lines. These high pressure lines are needed because if high pressure gas in the underground will enter the wellbore, high pressures on surface will be required to be able to transport this gas out of the well in a controlled manner. The reason for the high pressure lines are the methods and procedures needed up until now on how gas are transported (circulated) out of a well under constant bottom hole pressure. Until now it has not been possible to follow these procedures utilizing and exposing the main marine drilling riser with low pressure ratings to these pressures. Formation influx circulation from bottom/open hole has to be carried out through the high pressure auxiliary lines.
In addition to these high pressure lines, there might be a third line connected to the internal of the main drilling riser in the lower end of the riser. This line is often called the riser booster line. This line is normally used to pump drilling fluid or liquids into the main bore of the riser, so as to establish a circulation loop so that the fluids can be circulated in the marine drilling riser and in addition to circulation down the drill pipe up the annulus of the wellbore and riser to surface. The drilling riser is connected to the subsea BOP with a remotely controlled riser disconnect package often defined as the riser disconnect package (RDP). This means that if the rig looses its position, or for weather reasons the riser can be disconnected from the subsea BOP so that the well can be secured and closed in by the subsea BOP and the rig being able to leave the drilling location or free to move without being subjected to equipment limitations such as positioning or limitation to the riser slip joint stroke length.
Generally, when drilling an offshore well from a floating rig or Mobile Offshore Drilling Unit (MODU), a so called “riser margin” is wanted. A riser margin means that if the riser is disconnected the hydrostatic pressure from the drilling mud in the borehole and the seawater pressure above the subsea BOP is sufficient to maintain an overbalance against the formation fluid pressure in the exposed formation underground. (When disconnecting the marine drilling riser from the subsea BOP, the hydrostatic head of drilling fluid in the bore hole and the hydrostatic head of sea water should be equal or higher than the formation pore pressure in the open hole to achieve a riser margin.) Riser margin is however difficult to achieve, particular in deep waters. In most case it is not possible due to the low drilling margins (difference between the formation pore pressure and the strength of the underground formation exposed to the hydrostatic or hydrodynamic pressure caused by the drilling fluid)
Managed pressure drilling (MPD) methods have been introduced to reduce some of the above mentioned problems. One method of MPD is the Low Riser Return System (LRRS). Such systems are explained in patent PCT/NO02/00317 and NO 318220. Other earlier reference systems are U.S. Pat. No. 6,454,022, U.S. Pat. No. 4,291,772, U.S. Pat. No. 4,046,191, U.S. Pat. No. 6,454,022.